This invention relates to a Nyquist filter for digital modulation generating a digital modulation waveform signal such as a QPSK (quaternary phase shift keyed) signal, a .pi./4-shift QPSK signal, or a 16-QAM (16-value quadrature amplitude modulation) signal.
Some Nyquist filters for digital modulation include ROM's (read-only memories). In digital communication, it is essential to prevent intersymbol interference. Since Nyquist filters using ROM's can be easily made into filtering characteristics well suited to remove intersymbol interference, such Nyquist filters are widely used in digital communication.
As will be explained later, a prior art Nyquist filter has some problem.